I like someone, do I ask the person if they like me?

It really depends on how long you've known the person. If you never really spoken to them before then all the sudden, you like them, then you really can't expect them to like you back.

If you do know the person though to a point where you feel comfortable, I would hold off asking if they like you. You should look at there body language when they are talking to you. For the most part, women have a more keen sense in reading body language than men. Body language is key and if you are not sure, then use Google and look up different body languages given off by a person who is interested. Flirting is a good way of measuring if he/she is interested. Some examples a guy might show interest, eye contact, flirtatious, eye contact with a smile, asks you questions, makes jokes, calls you when you ask him to, and etc.

This is such general question, it is difficult to answer unless
more details were given.

Hopefully that helps,

Gary